1. Field of the Invention
This invention relates to a disk reproducing device such as a CD (compact disk) player for optically reading information stored on a disk, and more particularly to a focus balance automatic adjusting device and automatic adjusting method for adjusting the focus balance by controlling a focus servo.
2. Description of the Related Art
In order to effect the recording/reproducing operation with high density and high fidelity, a digital storage/reproduction system for converting an audio signal into a digital signal by the PCM (Pulse Code Modulation) technique, storing the signal on a recording medium such as a disk or magnetic tape and reproducing the signal is used. Particularly, a CD player for creating a bit (or pit) string corresponding to digital data on a disk with a diameter of 12 cm and optically reading the bit string is now most widely used. With such a disk reproduction device, recorded data is read by moving an optical pickup unit having a semiconductor laser, photoelectric converting element or the like disposed therein in a direction from the inner periphery to the outer periphery of the disk by the linear tracking and rotating the disk with the constant linear velocity (CLV). When a semiconductor laser is used as the pickup element, a laser beam is applied from the pickup element to the track on the disk to detect the presence or absence of a pit based on the intensity of the reflected light and recorded digital data is reproduced based on the result of detection.
In order to always create the smallest spot by the laser beam on the track of the disk on which data is recorded, it is necessary for the pickup element to follow the fluctuation of the focus position caused by a variation in the thickness of the disk, the warping thereof and the like. As the error detection method of the focus servo to serve the above purpose, a large number of methods such as an astigmatism method, knife-edge method, wedge prism method and critical angle method are known. Among the above methods, the astigmatism method is most often used. In the astigmatism method, a plane-parallel plate or cylindrical lens is disposed in a return path of the laser beam or focusing light path of the reflected beam to intentionally cause a large astigmatism, detect an elliptical beam form before and after the circle of least confusion (circular beam form) and derive a focus error signal. If a detector is disposed in the position of the circle of least confusion with the disk set in the focused position, the light amount distribution on the detector is changed into an elliptical form in the out-of-focus position. Therefore, a focus error signal can be obtained by adding the detection signals on portions of the respective diagonal lines of the four-divided detector and deriving the difference therebetween. This method has an advantage that the optical system can be made small and the detection sensitivity can be made high.
Next, the focus balance adjusting circuit of the focus servo for the optical system of the conventional disk reproducing device using the astigmatism method is explained in detail with reference to FIGS. 1 and 2.
In the circuits shown in FIGS. 1 and 2, the above-described four-divided detector is used. A four-divided detector 11 constructed by a photodiode, for example, is designed to permit signals (currents) to be derived from four divided portions. The signal currents on portions lying on the respective diagonal lines are added together and the results of addition are respectively supplied to current-voltage converters 12-1 and 12-2 and respectively supplied to an inverting input terminal (-) and non-inverting input terminal (+) of a differential amplifier 13 via resistors R1 and R2. A difference between the two signals output from the detector 11 is derived by the differential amplifier 13. A parallel circuit of a resistor R3 and a capacitor C1 is connected between the inverting input terminal (-) and an output terminal of the differential amplifier 13. Further, a capacitor C2 and a semi-fixed resistor (volume) 19 is connected between the non-inverting input terminal (+) of the differential amplifier 13 and a ground terminal. An output of the differential amplifier 13 is supplied to a focus equalizer 15 as a focus error signal FE and the gain and phase thereof are compensated for by the equalizer 15. Whether the focus error signal FE is supplied to the equalizer 15 or not is controlled by use of a loop ON/OFF switch 14. When the switch 14 is set in the OFF state, the focus error signal FE is supplied to the equalizer 15, and when the switch 14 is set in the ON state, the potential of the input terminal of the equalizer 15 is set to the ground potential. An output signal of the focus equalizer 15 is supplied to a selection switch 16. The selection switch 16 is switched to select the output signal of the equalizer 15 or a focus search signal FS and supplies a selected signal to a driver 17. Then, a focus actuator 18 is driven by the driver 17.
Output signals of the current-voltage converters 12-1 and 12-2 are supplied to an inverting input terminal (-) of a differential amplifier 20 via resistors R4 and R5, respectively. A resistor R6 is connected between the inverting input terminal (-) and an output terminal of the differential amplifier 20 and the non-inverting input terminal (+) thereof is connected to the ground terminal with this connection, the differential amplifier 20 acts as an adder. A total sum signal of the four-divided detector 11 is derived by the adder, and as a result, an RF (radio frequency band) signal (which is also called as a HF signal) which is information data is created.
The RF signal is binary coded by a data extraction circuit 22 and supplied to a signal processing circuit 23 as an EFM (eight to fourteen modulation) signal. In the signal processing circuit 23, the EFM demodulation, sub-code demodulation, error correction processing and the like are effected and the results of the processings are supplied to a digital/analog converter (DAC) 24 as data DA. An output of the DAC 24 is supplied to a low-pass filter (LPF) 25 and the high frequency component thereof is cut off. Then, an output of the LPF 25 is output as a reproduced audio signal A.sub.OUT.
In the focus balance adjusting circuit shown in FIG. 1, the focus balance adjusting operation is effected by use of the semi-fixed resistor (volume) 19. That is, the focus balance is adjusted by connecting the semi-fixed resistor 19 to one of the signal paths via which two signals obtained by adding outputs of portions on the respective diagonal lines of the four-divided detector 11 are supplied to the differential amplifier 13 and changing the gain of the differential amplifier 13. Generally, a target to be adjusted is not determined and varies depending on the type of the pickup and the error detection method. Since the performances of recent pickups are made stable, a method for setting the target to the jitter minimum point of the RF signal and amplitude maximum point may be often used. Further, the RF signal is subjected to the EFM, but a method for extracting only the signal component called 3T which most frequently occurs in the signal components of the RF signal by use of a band pass filter or the like and adjusting the extracted signal level to the maximum point can be used. Since the performance of an optical pickup head of a CD player or the like is stable, the focus balance adjustment can be effected by setting the adjusted point to a point at which the amplitude of the RF signal becomes maximum.
FIG. 2 shows another focus balance adjusting circuit of the focus servo in the conventional disk reproduction device using the astigmatism. The circuit shown in FIG. 2 is different from the circuit of FIG. 1 in that an offset adding section formed of a semi-fixed resistor 21A and a resistor 21B is connected to the inverting input terminal (-) of the differential amplifier 13. The circuit of FIG. 1 is based on a system for adjusting the gain of one of the differential inputs, and FIG. 2 is based on a adjusting system by offset addition. The other circuit construction is substantially the same as the circuit of FIG. 1 simply by replacing the semi-fixed resistor 19 by the resistor R7. However, in either case, it is always necessary to adjust the focus position at the time of focus servo operation in the manufacturing process.
As described above, the balance adjustment in the conventional focus servo system is effected by adjusting the gain of an amplifier (differential amplifier 13) for creating a focus error signal FE to control the degree of out-of-focus caused by variations in the detectors 11 and variations in the optical parts or controlling the offset amount applied to the focus servo system. That is, the focused position is adjusted by controlling the focus servo loop. The adjustment of the focused position is effected by reproducing a reference disk or the like on the manufacturing line at the time of mass production of sets and adjusting the semi-fixed resistor (volume) 19 or 21A.
However, in recent years, the effect obtained only by the above volume adjustment is not satisfactory from the viewpoint of the enhancement of the playability, omission of the adjustment of the set and maintenance of the performance against deterioration with time which are required for the disk reproduction device. Further, the focus balance adjustment has not been practiced since the automatic operation thereof is difficult from the viewpoint of the performance of the pickup. Therefore, the adjustment was effected by a robot or a person by reproducing a reference disk on the manufacturing line at the time of mass production of sets. As a result, the condition of the adjustment is limited to the condition set at a specified time and it is impossible to adjust a slight deviation of the adjustment point during the operation caused by deterioration of the constant of the parts and the pickup element and variation in the temperature environment.
As the conventional focus balance automatic adjusting device, devices disclosed in Japanese Patent Disclosures (KOKAI) No. H.2-118923 and H.3-3121 are known. The former device uses a detector for an amplitude of the RF signal sensitive to the de-focus to adjust the conversion gain of the first-stage differential amplifier. However, in general, since the de-focus characteristic displays a parabolic characteristic, a problem that the direction in which the differential gain should be changed cannot be determined at the starting time of the adjusting process occurs. Therefore, it becomes necessary to change the differential gain in one direction, detect a variation in the state caused at this time and determine the direction in which the feedback must be effected. As a result, the adjustment becomes complicated.
In the latter device, a process of monitoring the de-focus state at a plurality of points, detecting two points at which the amplitude levels of the VFO are always equal to each other, causing the controller to fetch the value obtained at this time and deriving an offset at the intermediate point between the two points based on the above information becomes necessary. That is, in the technique disclosed in the above document, it is necessary to detect the position which is de-focused at least once with a relatively high resolution even when the initial state is temporarily set extremely near the final adjustment point based on variations in the sets and long time for the adjustment is necessary.